TE-125 NUCLEAR-MAGNETIC-RESONANCE AND ELECTROCHEMICAL INVESTIGATION OF EXCHANGE AND REDOX REACTIONS OF ORGANOTELLURIUM(IV) DITHIOLATE AND ORGANOTELLURIUM(II) COMPLEXES OCCURRING IN SOLUTION AND AT ELECTRODE SURFACES

Tellurium-125 NMR studies on a series of xanthate (xan), dithiophosphate (dtp), and dithiocarbamate(dithiolate) (dtc) organotellurium(IV) complexes Ph2Te(S-S)2 [where [S-S]- = [S2COEt]-, [Etxan]-; [S2P(OEt)2]-, (Et2dtp]-; [S2CNEt2]-, [Et2dtc]-] formed in situ by reaction of Ph2TeCl2 and dithiolate ligand show their instability in solution with respect to metal reduction to give Ph2Te and ligand oxidation to give (S-S)2. The rate of this internal redox decomposition reaction increases in the order Etxan > Et2dtp > Et2dtc and is dependent on temperature and solvent. In contrast, mixed-ligand compounds Ph2Te(Cl)(S-S) formed by exchange reactions between Ph2TeCl2 and [S-S]- and the series of C8H8Te(S-S)2 compounds (C8H8 = o-xylene-alpha, alpha-diyl) appear to be relatively stable in solution. Exchange reactions between different Ph2Te(S-S)2 complexes also are observed on the synthetic time scale, but in this case the reverse order of reactivity is found with Et2dtc complexes reacting to a significant extent and Etxan only to a minor extent. Electrochemical investigation on the C8H8Te(S-S)2 and Ph2Te(S-S)2 complexes in dichloromethane confirms the importance of exchange reactions between tellurium(IV), tellurium(II), and ligand species as well as with mercury electrodes. For example, reduction of C8H8Te(S-S)2 at platinum electrodes follows the pathway C8H8Te(S-S)2 + 2e- --> C8H8Te + 2(S-S)-and occurs at very negative potentials. At mercury electrodes, the rapid intermolecular exchange process C8H8Te(S-S)2 + Hg reversible C8H8Te + Hg(S-S)2 occurs at the electrode surface and is followed by reduction of the mercury complex. The same overall products are produced as is the case with reduction at platinum electrodes. However, at mercury electrodes, substantial catalysis of the reduction process is achieved in the sense that the potential for reduction is more than a volt less negative than at platinum electrodes. Exchange reactions at mercury electrodes also influence oxidation processes. One of the processes observed at mercury electrodes in the presence of C8H8Te is 3C8H8Te + Hg reversible [(C8H8Te)3Hg]2+ + 2e- which involves oxidation of electrode mercury. Unfortunately, the electrochemical processes observed at chemically inert platinum electrodes are all completely irreversible, so that no information could be obtained on the thermodynamic relationships existing in the Te(IV)/Te(II) dithiolate chemistry.